(Ti,Al)N PVD coatings on cutting tools are well known in the art. Both homogenous coatings and coatings comprising a multiple of thin layers have been described in the art.
EP 983393 A1 describes an aperiodic multilayer structure of alternating nitride layers, e.g. TiN/TiAlN multilayers.
EP 1795628 A1 describes a coated cutting tool insert having a PVD coating comprising an aperiodic multilayered structure of alternating AlxTi1-xN and TiyAl1-yN layers, where x=0.4-0.7 and y=0.6-1 with an average thickness of one A+B-sublayer within the range 30-300 nm, preferably 60-120 nm.
Multilayered (Ti,Al)N PVD coatings usually have compressive residual stresses which increase with coating thickness. If the compressive residual stresses become too high, the coating will crack, especially close to the cutting edge. On the other hand, a thicker coating will have a higher wear resistance which will prolong the tool life.
There is an ongoing strive to try to improve the performance of multilayered (Ti,Al)N PVD coatings and there is always an aim to achieve thicker coatings with low compressive stresses.
It is therefore an object of the present invention to provide a multilayered (Ti,Al)N PVD coating having a lower compressive residual stress.
It is another object of the present invention to provide a multilayered (Ti,Al)N PVD coating which can have an increased thickness without cracking.
It is yet another object of the present invention to provide a method of making a multilayered (Ti,Al)N PVD coating.
It has now been discovered that the above objectives can be achieved by the present invention as described below.